A typical wind turbine is illustrated in FIG. 1. The wind turbine 1 comprises a tower 2, a nacelle 3 mounted at top of the tower 2 and a rotor 4 operatively coupled to a generator 5 within the nacelle 3. The wind turbine 1 converts kinetic energy of the wind into electrical energy. In addition to the generator 5, the nacelle 3 houses the various components required to convert the wind energy into electrical energy and also the various components required to operate and optimize the performance of the wind turbine 1. The tower 2 supports the load presented by the nacelle 3, the rotor 4 and other wind turbine components within the nacelle 3.
The rotor 4 includes a central hub 6 and three elongate rotor blades 7a, 7b, 7c of approximately planar configuration which extend radially outward from the central hub 6. In operation, the blades 7a, 7b, 7c are configured to interact with the passing air flow to produce lift that causes the central hub 6 to rotate about its longitudinal axis. Wind speed in excess of a minimum level will activate the rotor 4 and allow it to rotate within a plane substantially perpendicular to the direction of the wind. The rotation is converted to electric power by the generator 5 and is usually supplied to the utility grid.
A conventional rotor blade is made from an outer shell and an inner hollow elongate spar of generally rectangular cross section. The spar serves to transfer loads from the rotating blade to the hub of the wind turbine. Such loads include tensile and compressive loads directed along the length of the blade arising from the circular motion of the blade and loads arising from the wind which are directed along the thickness of the blade, i.e. from the windward side of the blade to the leeward side.
An alternative type of rotor blade is known which avoids the need for an inner spar by incorporating within the outer shell one or more fibrous reinforcing structures of high tensile strength which extend along the lengthwise direction of the blade. Examples of such arrangements are described in EP 1 520 983 and WO 2006/082479. Other arrangements are also described in US 2012/0014804 and WO 2011/088372.
In these arrangements, use is made of pultruded fibrous strips of material. Pultrusion is a continuous process similar to extrusion, wherein fibres are pulled through a supply of liquid resin and then heated in an open chamber where the resin is cured. The resulting cured fibrous material is of constant cross section but, since the process is continuous, the material once formed may be cut to any arbitrary length. Such a process is particularly cheap and therefore an attractive option for the manufacture of reinforcing structures for wind turbine blades.
The use of cured pultruded strips overcomes problems associated with conventional arrangements in which non-cured fibres are introduced into a mould to form parts of a wind turbine blade, in which there is a risk of the fibres becoming misaligned.
Furthermore, pultruded strips have the property of absorbing the very high bending moments which arise during rotation of wind turbine blades.
In the above two known arrangements, a relatively large number of separate elements are used to form the reinforcing structure, and each element must be individually positioned within the structure of the shell.
It would be desirable to provide a suitable reinforcing structure for a wind turbine blade of this alternative type which is of simpler construction and therefore cheaper to manufacture.
US 2009/0269392 describes a wind turbine blade comprising elongate structural members formed from laminated fibre cloths infiltrated with resin.
However, in this arrangement the fibre cloths are cured in situ which requires the cloths to be carefully positioned and correctly oriented on the surface of the shell prior to moulding.
It would therefore be desirable to provide a wind turbine blade which overcomes, or at least mitigates, some or all of the above disadvantages of known wind turbine blades.